I want to hear the entire thought process so I can discern how much the candidate really knows. If I was asked this question, I would say obviously the theoretical limit is 80 seconds (provided you use raw data with no compression) and there is no way it could be done any faster. Add to that frame + packet + TCP overhead, delays due to ack/handshaking + latency, packet loss and the download takes longer. Also, speed is influenced by MTU / Jumbo Frames settings, TCP window sizes, and tuning options.
I would have accepted the answer even if the candidates were slightly wrong or off the mark on any of these. Instead I got answers from "ummmmm" and "less than 1 second" to "35 minutes" and "it's impossible to answer this question." Like _delirium said, the basic answer is available on Google so it wasn't a single right/wrong answer that I wanted, of course that would help. I just wanted to hear them think this problem through. I even broke down the problem for them and told them 1 byte = 8 bits. So 10 MB = 10 million bytes and 10 Mbps = 10 million bits per second. sigh
This isn't just a random interview question by any means. I have a solid reason to ask this - we have hosted Exchange from Rackspace and because of how Terminal Server works, we have to use it in non-cached mode. My users use Outlook but it works pretty much like any web-based mail app does - every email is downloaded each time you click on it. It is worse because browsers can at least cache attachments. So a user who gets a 10MB file gets mad at me because it takes 30 seconds to download it. If the new IT person can't understand and explain why that happens, how can they determine the solution to this problem?